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Research Interests

My research program integrates ecological, comparative, and microevolutionary studies to understand the evolutionary trajectories of populations and the structure of communities in the wild. In particular, my lab focuses on four core areas: causes and consequences of interspecific hybridization, invasion biology, maintenance and effects of genetic diversity, and the evolution of genome size. We often use plants and their animal associates (herbivores, seed dispersers, pollinators, ant guards) as study systems. We employ a variety of techniques, including experimental field and greenhouse studies, techniques from molecular and statistical genetics, phylogenetic comparative methods, and chemical analyses of plant secondary compounds.

Current Projects

Hybridization, adaptation, and invasion

Historically, natural hybrids were seen as unfit, uninteresting, and ‘evolutionary dead ends.’ As this paradigm passes, we are investigating the hypothesis that hybridization can have dramatic effects on evolution because it introduces large pulses of novel genetic material. Our main study system is a naturally occuring sunflower hybrid, Helianthus annuus ssp. texanus, and its parental species, H. annuus and H. debilis. The hybrid has colonized central and southern Texas, making it an interesting case study of the potential link between hybridization and colonization success. We are comparing long-term (>10 year) evolutionary change in experimental hybrid and control (parental) sunflower lines in the field – in effect, recreating the hybridization event and replaying the evolutionary clock. Hybrid lines have rapidly evolve equal fitness to parents; strikingly, the hybrid lines are also showing signs of soon exceeding parental lineages in fitness and performance (e.g., herbivore resistance). Using a recently developed single nucleotide polymorphism (SNP) library, we are connecting hybrid success to individual quantitative trait locus (QTL) alleles derived from each parent, and plan to examine the changes in frequencies of these QTL alleles over time in the hybrid populations.

We have also begun exploring the links between hybridization, adaptation, and invasiveness in other plant systems, including Raphanus in Texas and Tragopogon in the Rocky Mountains. Studies of these specific hybrid systems have in turn led to a broader interest in natural patterns of hybridization in plants. My lab has compiled the largest dataset ever assembled of natural hybrids recorded in floristic treatments (>37,000 species accounts), and has used it to describe geographic and taxonomic patterns of hybridization and to ask whether hybridizing groups tend to produce invasive species. Currently we are testing whether plant traits (e.g., perenniality, range size, genetic divergence, and many others) are predictive of hybridization behavior, hoping to solve the long-standing riddle of why some plant groups hybridize more than others.

Sparked in part by our studies of hybridization and invasion (above), we are interested in the ecological and evolutionary determinants of biological invasions. Graduate student Amy Savage has shown that novel mutualisms with plants have in part determined the success and impacts of the invasive Yellow Crazy Ant in the Samoan Archipelago. We have shown that phylogenetic distances can be an important predictive tool in invasion biology, as they are related to the success of bird invasions and herbivore attack rates on exotic plants. Finally, we are examining the role of genetic diversity in colonization and invasion success. Genetic diversity of invasive/colonizing species was previously recognized as important only over longer-term evolutionary timescales, but work by graduate student Kerri Crawford is the first to show that it can have dramatic short-term ecological effects via increasing founder population establishment.

Genetic diversity has important population and community-level consequences, but how multiple genotypes are maintained in populations is an open question -- the genetic equivalent of asking how multiple species are maintained in ecological communities. I have investigated the maintenance of genetic diversity in the Australian desert shrub Acacia ligulata, which has a genetic polymorphism in carotenoid pigmentation: an individual plant produces either red, orange, or yellow arillate seeds. Both avian seed dispersers and (unexpectedly) insect seed predators mediate selection on fruit color, such that the identity of the most fit morph varies among populations. These studies were the first to demonstrate that animals can produce selection on fruit color in natural populations, and that fluctuating selection likely acts to maintain such polymorphisms. More recent work on genetic polymorphisms has focused on maintenance of chemical morphs in the widespread weed Xanthium strumarium. Graduate student Jeff Ahern has demonstrated that a stereochemical polymorphism in sesquiterpene lactones has large effects on herbivore resistance, and that the two stereochemical morphs perform differently in different sites, indicating a role for spatial variation in herbivores and other factors in maintenance of the chemical polymorphism.

Arillate seeds of the three Acacia ligulata color morphs

The enigma of genome size variation

Across the tree of life, genome size spans several orders of magnitude. This variation does not appear to be correlated with organismal complexity or with gene number, leading to what is called the 'c-value enigma': why do similar organisms with similar amounts of coding sequence have vastly different amounts of DNA? Ongoing work in the lab is challenging the widely accepted hypothesis that genome sizes expand through genetic drift associated with small population size. According to this model, taxa with small effective population sizes (Ne) have relatively large genomes because genetic drift is relatively strong (and natural selection is relatively weak) in small populations, allowing extra DNA to accumulate. However, our examinations of both seed plants and a taxonomically diverse data set have shown no support for this paradigm, instead suggesting that artifacts arising from phylogenetic history create a non-causal correlation between Ne and genome size. Other areas of interest include the ecological consequences of polyploidy (genome duplication) and whether hybridization can trigger changes in genome size.

Evolution of genome sizes across a sample of bacteria, plants, fungi and animals

How frequently, and under what conditions, does evolution repeat itself? When populations do independently evolve the same characteristics, are the underlying genetic changes similar or different? Advances in genetic techniques are finally allowing answers to these fundamental questions about the way evolution works. However, these issues have not yet been explored in hybridizing lineages, despite the fact that hybridization (mating and cross-fertilization between different species) is widespread in wild plants, animals, and fungi. This project examines replicate populations of experimental hybrid sunflowers which have been evolving in the field in Texas. Morphological and physiological measurements will determine whether they are converging or diverging in their traits, and new genetic sequencing techniques will be used to determine whether the genetic basis of the trait change is similar or different across the replicates. This research is the first to experimentally examine the repeatability of hybrid evolution and characterize its genetic architecture in the wild. It will answer fundamental questions about the way that evolution produces biodiversity, and has the potential to inform plant and animal breeding.

Effects of population genetic diversity on colonization success

NSF DEB 1146203; co-PI Stephen Hovick; 2/2012 - 1/2015; $310,822

Colonization is a critical process that affects both short-and long-term population dynamics and sets the stage for future evolutionary change. Colonization success is affected by many factors, among the most important of which is propagule number. However, propagule number is highly correlated with genetic diversity because introducing more propagules generally samples the source gene pool more completely. Theoretical considerations suggest that increased genetic diversity should enhance colonization success independently of propagule number, yet few experiments have investigated this issue and only one field study has decoupled propagule number from genetic diversity. The proposed research will assess the extent to which genetic diversity interacts with propagule number and variation in neighbor abundances to influence colonization success in plants, using as a framework recruitment functions such as the Beverton Holt. High genetic diversity is predicted to 1) enhance maximum colonization success, 2) enhance the per-propagule effect on colonization success at low propagule densities and 3) benefit founder populations in highly competitive environments more so than in less competitive ones. This study will examine the mechanisms underlying the effects of genetic diversity, testing whether sampling effects and complementarity shift in relative importance as propagule pressure increases and interspecific competition is reduced, and whether the importance of these two factors varies over time.

Hybridization is a widespread phenomenon, yet its role in evolution is still under debate. Is it a maladaptive, homogenizing force (think mules) or can it contribute to adaptation and evolutionary diversification? We are currently implementing a novel approach that compares long-term evolutionary change in experimental hybrid and control (non-hybrid) lines in the field. These lines are modeled on (i.e. derived by crossing the parents of) a well-studied hybrid sunflower lineage, thus providing a rich context for interpretation. The proposed project asks: (1) Can introgression increase rates of adaptation?, (2) Can introgression increase rates of phenotypic evolution?, and (3) Are evolutionary trajectories in hybrid populations predictable? These questions will be addressed by tracking fitness, 20 traits, and 20 molecular markers (linked to quantitative trait loci, QTL) in the experimental hybrid and control populations over 5-10 generations. Evolutionary change will be distinguished from phenotypic plasticity by comparing the lines in replicated common gardens. The long-term predictability of change in hybrid systems will be examined by assessing whether the experimental hybrids converge phenotypically and genotypically on the natural hybrid upon which they are modeled. The proposed research is first experimental field study to examine the impact of hybridization on adaptive evolution over multiple generations in a wild (non-crop) system.

Plants can both inhibit each other chemically (allelopathy) and exchange information via volatile organic compound emissions. While most research in this field has focused on interactions between adult plants, tantalizing reports indicate that seed-seed chemical interactions do exist. While some seed-seed interactions are apparently allelopathic, in some cases a seed may sense the environment and respond in an adaptive way, by either accelerating or delaying germination. While seed-seed signaling is potentially of great importance to both plant developmental processes and the structure of ecological communities, we know nothing about the genetic and chemical pathways involved, nor how often such signaling has evolved under the “allelopathy” scenario, the “sensing” scenario, or both. Using our recent discovery of putative seed-seed signaling in Arabidopsis thaliana , this project will attempt to (1) elucidate the molecular pathways involved in seed-seed signaling, and (2) develop the plant material and techniques allowing tests of the role of seed-seed signaling in evolutionary adaptation and in structuring ecological communities.

Invasive species pose one of the greatest threats to global biodiversity, and tropical oceanic islands are particularly vulnerable to their negative impacts. For these systems, invasion by the yellow crazy ant (Anoplolepis gracilipes) is a major threat. Identified by the International Conservation Union as one of the world's 100 worst invaders, this species has already decimated some tropical island ecosystems. In Samoa, an island group integral to the Polynesia/Micronesia biodiversity hotspot, presence of the yellow crazy ant is of acute concern. Our data suggest that yellow crazy ants are at a critical stage in their invasion, possibly transitioning from low-level persistence into a phase of rapid population growth with potentially severe ecological consequences. We will investigate the ecological mechanisms that underlie yellow crazy ant success, examine early impacts of the invasion on native communities, and test how community dynamics, specifically novel beneficial relationships with native species, may feed back to influence the invasion. This work will both advance ecological theory and provide critical information needed for conservation planning.